I am an atheist. (Specifically, a humanist.) I don’t believe in any god.
However, I do acknowledge that my knowledge is limited. Everybody’s knowledge is limited (as far as I can tell). There could be all sorts of weird things out there that I’m not aware of, including things that I’ve never even imagined. It’s possible that there really is a God out there somewhere, and I just haven’t found him yet. Maybe there are several gods. Who knows?
Maybe the Christians are right. Or the Muslims or the Hindus. Or maybe they’re all wrong, and the real truth is completely different from everybody’s expectations. (If there really is a god, I’m guessing that he fits into the “all major religions are wrong” category.)
This applies to plenty of other concepts too. Maybe there’s an afterlife, for instance, and I just haven’t discovered it yet. (Maybe we eventually create an afterlife with technology somehow.)
If I find enough evidence to prove the existence of a God, or an afterlife, or unicorns or anything else, then I will change my beliefs.
So you see, just because I’m atheist doesn’t mean that I’m not open to new ideas. And it doesn’t mean that I’m 100% certain that “this is all we’ve got”. There’s always room for new discoveries, and there’s always room for hope.